The Long Term Evolution (LTE) Physical Downlink Control Channel (PDCCH) is used for transmitting scheduling grants, to grant uplink transmissions, and downlink assignments, to inform a User Equipment (UE) where and how the downlink data is transmitted. These messages are unique for each subframe (1 ms) in LTE.
In the present context, the expression “downlink” is used to specify the transmission of wireless signals from the base station, which also may be referred to as an eNodeB or eNB, to the user equipment, while the expression “uplink” is used to denote the transmission from the user equipment to the base station.
The resources available for PDCCH are limited and more resources are consumed by a message if the corresponding downlink quality is bad.
In LTE there is also a limitation in that each user only monitors parts of the available PDCCH resources, so called “search space limitation”. This means additional restrictions in the PDCCH resource allocation.
Thus, all in all, for scenarios when there are many mobiles to schedule, the number of available PDCCH:s each Transmission Time Interval (TTI) may be a bottleneck, and thus limiting the cell capacity.
A PDCCH message can be transmitted using 1, 2, 4 or 8 control elements, such as e.g. Control Channel Elements CCEs. The number of CCE:s needed to transmit a message depends primarily on the link quality. The total number of CCE:s available for PDCCH in each sub-frame is defined mainly by the carrier bandwidth and the number of Orthogonal frequency-division multiplexing (OFDM) symbols, dynamically configured for PDCCH.
The number of CCEs used for transmission of a particular PDCCH is determined by the base station according to the channel conditions. If the PDCCH is intended for a user equipment with a good downlink channel, i.e. situated close to the base station, then one CCE may be sufficient. However, for a user equipment with poor channel condition e.g. situated near the cell border, then eight CCEs may be required in order to achieve sufficient robustness. In addition, the power level of a PDCCH may be adjusted to match the channel conditions.
Scenarios where PDCCH may limit the capacity of the wireless communication system comprise many simultaneous users transmitting small packets, which may be the case e.g. in Voice over Internet Protocol (VoIP). An example of a scenario where PDCCH limitation is likely is a voice-only scenario with high load i.e. many users.
One possible method to combat the PDCCH limitation for VoIP is to use Semi-Persistent Scheduling (SPS), where one single PDCCH message can be used for allocating data resources with a predefined repetition pattern in time.
When using semi-persistent scheduling for VoIP, a periodic downlink transmission resource is allocated during a talk-spurt on the downlink. The same resource is allocated each time with a given periodicity. The allocation is turned on during each of the talk-spurts and off between talk-spurts, i.e. in silence. Thereby, explicit signalling to request an allocation and to grant a particular VoIP allocation is not required during talk-spurts. Semi-persistent scheduling for uplink VoIP communications from a user equipment is similar.
However, semi-persistent scheduling still needs to use PDCCH resources to signal the semi-persistent allocation, release it, change modulation and coding, increase or decrease the number of resources blocks to be used, or move the allocation. There is also often a need for explicit PDCCH messages for re-transmissions due to collisions with semi-persistent scheduling allocations. Furthermore, efficient use of Disrupted Transmission (DTX) periods must be explicitly signalled. Thus, efficient usage of PDCCH resources is important both for dynamic and semi-persistent scheduling.